This invention relates to the control of a vehicle speed and is particularly applicable to vehicles having engine control systems with anti-stall ability.
4xc3x974 xe2x80x9coff roadxe2x80x9d vehicles may be equipped with anti-stall systems to prevent the vehicle stalling when traversing surfaces which resist vehicle motion, for example mud and sand. Such systems give the driver confidence when driving off-road and reduces the demands on the driver through clutch and throttle co-ordination.
However, anti-stall systems have a disadvantage in relation to vehicle control in that the system will not allow the vehicle engine speed to drop below that required to maintain the set idle speed, when a driver operates a braking system or a hill descent control system (HDCS). Such a situation may arise during the application of HDCS, or during the application of the brakes when traversing an obstacle. In such situation the idle speed controller xe2x80x9cfightsxe2x80x9d the braking system, with the idle speed controller increasing engine torque to maintain idle engine speed (and hence road speed) and the braking system or HDCS having to apply an increasing braking load to slow the vehicle down.
The situation arises because some 4xc3x974 off-road vehicles are not provided with a low ratio gearbox and road speed associated with the idle set speed may be relatively high. For Example, with an idle speed set point at about 1000 rpm and the vehicle in first gear, this may give a ground speed of up to 10 kph. This idle speed set point may be increased by the engine speed controller for cold starting, operation of air conditioning systems, increased battery loads, etc.
The present invention provides an improved vehicle speed control system.
According to a first aspect of the present invention there is provided a vehicle speed control system having an engine control system with an engine idle speed controller which maintains the engine idle speed at varying engine torque outputs, and a vehicle braking system for braking the vehicle and which is operated by a deceleration demand means wherein the idle speed controller is arranged to receive signals from the deceleration demand means and any increase in engine torque output demanded by the idle speed controller is limited in response to signals received from the deceleration demand means when the braking system is operational.
The braking system may include at least one of a wheel braking system and a transmission braking system which are activated by the deceleration demand means.
The deceleration demand means includes at least one of a driver operable brake pedal, a HDCS, or a stability control system, each of which produces a respective deceleration demand signal and the actual permitted increase in engine torque output may vary depending upon the source of a deceleration demand signal.
When the deceleration demand signal is derived from a driver operated brake pedal for a wheel braking system, the vehicle driver will experience through the brake pedal an increase in braking effort due to an increase in engine torque output which indicates to the driver that the vehicle is being slowed and that the idle speed may drop below the idle speed set point. This increase is limited to minimise additional brake wear, heat input, and braking effort.
When the deceleration demand signal is derived from a stability control system or the HDCS, then the increase in engine torque output is reduced to a minimum so that no excess effort is required from the braking system. This has the benefit of reducing brake wear, increases engine durability, reduces fuel usage and exhaust emissions.
The limited lower torque output demand is not activated instantaneously but is applied progressively so that no instantaneous changes in produced engine torque are generated, provided that said lower torque demand is above the actual torque demand at that instant. This is particularly important when a vehicle ascending a hill in low gear with the engine operating at the idle set point and the driver needing to operate the vehicle brakes. If the lower limit were applied immediately and the engine torque reduced the vehicle may stop and even roll back. With a progressive reduction in torque the vehicle will slow and the driver can take corrective action e.g. increase the throttle torque demand.